Recycling of asphaltic concrete has become increasingly important due to the steadily increasing costs of asphalt and aggregate which make up the composition. Methods and apparatus utilized in such a processing include those disclosed in my U.S. Pat. Nos. 3,849,040, 3,999,743, and patents to others such as 4,075,710, 4,147,436 and 4,165,184. Although the latter two patents speak generally of treating exhaust gases from the process in conventional dust filtration and collection apparatus prior to being vented to atmosphere, none of the methods disclosed adequately treat such exhaust gases to meet strict environmental pollution standards.
Where asphaltic concrete is recycled according to presently preferred technology, it is taken up from the road or highway surface and crushed into a variety of particle sizes, such as disclosed in my aforesaid U.S. Pat. No. 3,999,743. It is then introduced into an elongated drum into which flame and hot gases of combustion are directed, thereby exposing the asphalt containing particles to the hot gases, in order to heat the composition and make it more plastic. Make-up asphalt and/or softening agents as disclosed in my U.S. Pat. No. 4,000,000 are also usually mixed with the molten material during processing. Normally, the asphaltic concrete recycle processes result in composition recovered having temperatures of above 200.degree. F., and usually above 225.degree. F., and product temperatures of 240.degree. F. or more are not uncommon. When the recycle product mixture is heated to such temperatures, substantial amounts of volatile and combustible hydrocarbon gases are given off by the hot asphalt. Such hydrocarbon volatiles mixed with the hot exhaust gases and at temperatures equal to or greater than the product temperatures, are of sub-micron size, and are simply not removed by conventional dust collectors, or even scrubbers, prior to being vented to atmosphere. Moreover, these volatile asphalt hydrocarbons are quite visible, as well as being generally undesirable as atmospheric pollutant materials.
In my prior U.S. Pat. Nos. 4,104,736 and 4,153,471, there are disclosed processes and apparatus for treating exhaust gaseous mixtures from such recycle operations as described hereinabove. Although these processes for treating the exhaust gases are useful, their apparatus features do not lend themselves for being easily adopted for use with conventional dust collecting and/or scrubbing equipment associated with the drum heating and mixing apparatus. It is to the improvement of known asphaltic concrete recycling appratus, particularly to modification of the apparatus for handling and treating exhaust gases containing combustible asphalt hydrocarbon volatile gases, that the present invention is directed.